Francis Galton, Darwin’s cousin and the father of eugenics, in the concluding chapter of his Inquiry into Human Faculty, in which he first named the science of eugenics:

It would be easy to add to the number of possible agencies by which the evolution of a higher humanity might be furthered, but it is premature to do so until the importance of attending to the improvement of our race shall have been so well established in the popular mind that a discussion of them would be likely to receive serious consideration… 

To sum up in a few words. The chief result of these Inquiries [eugenic science] has been to elicit the religious significance of the doctrine of evolution. It suggests an alteration in our mental attitude, and imposes a new moral duty. The new mental attitude is one of a greater sense of moral freedom, responsibility, and opportunity; the new duty which is supposed to be exercised concurrently with, and not in opposition to the old ones upon which the social fabric depends, is an endeavour to further evolution, especially that of the human race.

The rationale for eugenics was the perceived need to take control of human evolution. Eugenics — human husbandry — was based explicitly on evolutionary theory, in the words of its founder.